1. Field of the Invention
This invention relates to polyurethane compositions and, more particularly, to expanded polyurethane compositions and molded articles made from them.
2. Prior Art
Expanded polyurethane compositions are conventionally prepared by reacting one or more polyol compounds, i.e., polyhydroxyl-containing compounds, such as polyester polyols and polyether polyols with a polyisocyanate in the presence of moisture, the polyisocyanate component being present in excess of the stoichiometric amount to provide for the production of carbon dioxide, which is used as an expanding agent, by reaction of the excess polyisocyanate with the moisture. Expanded polyurethanes are also made by curing a polyurethane prepolymer in the presence of an external expanding agent, as by extruding the prepolymer through the barrel of an extruder equipped with one or more inlet parts to permit the introduction of the expanding agent, e.g., gaseous blowing agents, such as carbon dioxide; volatile solvents, such as halogenated hydrocarbons; volatile hydrocarbons, such as pentane; or chemical blowing agents, such as N,N'-dinitrosopentamethylenetetramine into the molten prepolymer as it passes through the pressurized barrel of the extruder. The molten material expands upon being discharged from the nozzle of the extruder barrel as a result of the lower pressure outside of the extruder. These methods present the disadvantages that either monomeric isocyanates must be used with their attendant hazards or an external expanding agent must be incorporated into a molten prepolymer.
United Kingdom patent specification No. 1,028,908, published on May 11, 1966 discloses, as an invention, the incorporation of a polyisocyanate into a monomeric reaction mixture containing hydroxyl groups which are available for reaction with the polyisocyanate to produce carbon dioxide and thereby produce a cellular product. This patent also discloses an embodiment in which a polymer is present. The invention in this patent, like that of the above described procedures, requires the handling of highly toxic isocyanates and liquid reactants.
Because of the above and other disadvantages of prior art procedures, science and industry researchers are continuously searching for methods of producing controlled density polyurethanes which avoid the nuisance and hazards of handling molten and/or toxic materials.